In various communication uplink systems, signals at the output of a receiving antenna vary from a maximum value of +60 dBm to a minimum of -15 dBm. Receiver circuitry connected to such an antenna is thus required to process signals over a 75 dB dynamic range. A good IF limiter could satisfy such dynamic range requirements, however uplink receivers in the past have not used the IF limiter in this way. In the prior art, signals from an antenna have first entered an RF limiter which then passed the signal to a fixed 45 dB attenuator. The output from the attenuator would then enter a low level mixer together with the output from a local oscillator. The output from the low level mixer would represent a dynamic range considerably less than the 75 dB characteristic of the signal at the output of the antenna. The output of the low level mixer was then entered into the IF limiter. By the time the signal entered the IF limiter, much of the upper portion of the dynamic range--which was within the limits of the IF limiter--was lost in the systems of the prior art.
The loss of dynamic range is attributable to the restrictions inherent in the low level mixer and the IF limiter when used together: the low level mixer having a maximum allowable input of 0 dBm with a conversion loss of 7 dB and the IF limiter being restricted in dynamic range between -70 dBm and 30 15 dBm by its design. As a result of these limitations, prior art schemes have not only failed to maximize the dynamic range capabilities of the IF limiter, but also required the use of an RF limiter in order to restore at least some of the upper portion of the signal that would be lost after the 45 dB attenuation and the low level mixing. Without the RF limiter, the +60 dBm signal would be attenuated 45 dB to +15 dB which is not an allowable input to the low level mixer. All signals between 0 and +15 dBm would thus be lost in the low level mixer stage. The RF limiter, in effect, transforms the "+60 dBm to -15 dBm" range into a "+45 dBm to -15 dBm" range, lowering and compressing the signal from the antenna to better fit the 45 dB attenuator and the low level mixer.
U.S. Pat. Nos. 3,185,931, 3,292,093, and 3,387,220 represent various prior art circuits which demodulate square FM signals into a corresponding sinusoid. U.S. Pat. No. 3,185,931 is not especially directed to mark-and-space data and does not vary the sinusoidal period depending on whether a mark is followed by a space or another mark. Similarly, U.S. Pat. No. 3,292,093 does not provide distinct but varying sinusoid outputs depending on a mark or space data sequence. U.S. Pat. No. 3,387,220 does disclose the demodulating of mark and space data into an alternating signal waveform, the varying of the waveform period depending in some way on the data; however, this reference teaches a three-level output signal without clear sinusoid characteristics and does not disclose two distinct waveform characteristics depending on what data follows a mark. None of the above references (nor any other known prior art) propose the elimination of RF limiting in a receiver system by using high level mixing. Such references thus fail to detect signals--with minimal noise, nonlinearity, and other distortion--over the full dynamic range of the receiver.
In general, the use of an RF limiter and the failure to exploit the full dynamic range of the receiver and its IF limiter does not especially adversely affect the operation of a communication receiver. In applications where waveform distortions and transients are particularly undesirable, however, the use of an RF limiter, which alters the initial signal, creates problems which make its use impractical.